A well known rule of thumb for a uniformly wound, circular cylindrical coil is that magnetic field strength at the winding ends approach half of the magnetic field strength at the center of the coil. Generally ovoid or rectangular-shaped wire windings made to receive rectangular-shaped magnetic media, such as computer memory devices like hard drives, suffer somewhat less field strength loss at the winding's ends. Two approaches to increasing the field strength at the ends of a coil include increasing the field strength at the center of the coil or increasing the coil length to be much greater than a degaussed medium's length; however, both approaches incur high energy storage and delivery costs.
Numerous prior art examples of bulk degaussers with magnetic field generating coils having more or less rectangular-shaped or ovoid windings with a tight, uniform pitch are known in the art. Nonuniform pitch windings have also been employed with noncircular coils to improve uniformity or to adjust electrical properties of the windings. Multiple windings have been used to gain magnetic strength or to provide directional variance by superposition of magnetic fields generated by individual windings.
The prior art of interposing a uniform winding with one or multiple gaps toward the middle of the windings works well for multi-filar or multi-wire windings stacked in parallel such that the effective turns pitch is shorter than the coil thickness. Introduction of gaps in windings where a filar has a more or less round or square aspect allows leakage magnetic flux that degrades uniformity. Such prior art multiple filar layers, however, increase the volume that needs to be magnetized.
Moreover, using multiple windings to allow magnetic field superposition always must increase the magnetic field volume significantly, which in turn requires increased energy storage and delivery requirements that increase with the square of magnetic strength. Increased volume also implies longer length of the coil wires and an increase in wasteful stray resistance over the wires that impacts quality factor. For example, if resistance is high enough to cause damping just over criticality, only a third of the energy stored in a capacitor can be delivered to generate a magnetic field. Other factors impacting the energy used include the saturation effects of nonlinear ferrous materials and stray resistive, capacitive, and hysteresis losses.
When applied to the task of degaussing hard disk drives, magnetic fields should be produced at a strength sufficient to erase all data stored on a respective drive. Hard disk drives currently have coercivities in excess of 5000 oersteds. In the near future, media will be produced that is designed for heat assisted magnetic recording, designed with patterned domains, or a combination of both with room temperature coercivity in excess of 10,000 oersteds. Magnetic field generators capable of generating a 20,000 oersted magnetic field intensity are becoming available, and may be incorporated with circuitry that can generate that strength in both axial directions of a coil. Generating such a magnetic field with uniformity sufficient to ensure erasure of the entire hard disk, however, has not been shown without having the drawbacks shown above.
Recent advances in the magnetic recording arts place less relevance on generating magnetic fields in multiple directions with respect to media. For one example, perpendicular media is formulated with a magnetically soft under-layer that is predisposed to erasure in response to exposure to any single magnetic field direction. For another example, media rotation confinement by parked heads is dubious in view of the likely action of strong externally applied magnetic fields on spindle motors. As a result, market acceptance is growing for bulk degaussers with unidirectional magnetic field generators, and windings can be produced to generate smaller and more uniform magnetic field volumes.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions and/or relative positioning of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of various embodiments of the present invention. Also, common but well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments. It will further be appreciated that certain actions and/or steps may be described or depicted in a particular order of occurrence while those skilled in the art will understand that such specificity with respect to sequence is not actually required. It will also be understood that the terms and expressions used herein have the ordinary technical meaning as is accorded to such terms and expressions by persons skilled in the technical field as set forth above except where different specific meanings have otherwise been set forth herein.